1. Field of the Invention
The present invention relates to an adaptive resending request control system in mobile radio communications for controlling the data transmission speed in mobile radio communications in accordance with the state of a communication line to achieve the optimum transmission speed and delay characteristic that satisfy the required quality.
2. Description of the Prior Art
In conventional mobile radio communications, an error correction coding system for carrying out self-correction (hereinafter referred to as “FEC” (forward error correction)) and an automatic repeat request system for resending data from a transmission side when a data transmission error occurs at a reception side (hereinafter referred to as “ARQ” (automatic repeat request)) are known as a method of implementing error free data transmission. Further, a hybrid ARQ system comprising the combination of FEC and ARQ is also known.
For example, Japanese Laid-open Patent Publication No. Hei-7-67175has proposed, as a prior art, a system of controlling the transmission speed stepwise in accordance with the line state (the line condition). FIG. 1 is a flowchart showing the control operation in this system.
Data transmission is started in control step 32, and it is judged in control step 33 whether the transmission speed is equal to the highest speed (9600 bps) or not. If the judgment in the control step 33 is “YES”, then it is judged in control step S34 whether there is any resending request or not. If there is a resending request, the resending processing is carried out in control step 35 and the resending frequency (the number of times of the resending) is counted. If it is judged in control step 36 that the resending frequency is equal to N or more, the transmission speed is decremented by one level in control step 37 and set to 4800 bps. Subsequently, resending frequency CNT is cleared in control step 38, storage memory is accessed in control step 39 and mode setting is carried out in control step 40. It is judged in control step 41 whether the transmission is finished or not.
If the judgment in the control step 33 is “No”, it is judged in a control step 42 whether correction is impossible. If the judgment in the control step 42 is “Yes”, it is judged in control step 44 whether the transmission speed is equal to 4800 bps which is lower than 9600 bps by one level. If the judgement in the control step 44 is “Yes”, a resending request is made in control step 45. On the other hand, if the judgment in the control step 44 is “No”, it is judged whether the transmission is finished or not as in the case of the control step 41. If the judgment in the control step 42 is “No”, the correction processing is carried out as in the case of the control step 43.
In this system, when a burst error occurs, control steps of several stages are needed until the transmission speed reaches the optimum one because the transmission speed is stepwise controlled. Accordingly, this system has a disadvantage that the control thereof cannot sufficiently follow occurrence of a burst error. Further, the conventional technique has the following disadvantage. That is, since the resending control period is fixed, the delay time of the data resending cannot be controlled and thus the optimum delay characteristic cannot be achieved in accordance with the line state. Further, there is no variation in the measuring method of the line state, and thus high-precision measurement cannot performed on some line states.
Furthermore, Japanese Laid-open Patent Publication No. Sho-64-42944 discloses a device for detecting a bit error rate by line quality detecting means and varying the coding rate in accordance with the detection result in a packet communication device having an error correcting function. According to this technical concept, the coding rate or the type of the coding is varied in accordance with the line quality, and if the line quality is excellent, the information amount is increased to achieve an effective use of the line. According to this system, the information speed is variable, however, the transmission speed is fixed.
Still furthermore, Japanese Laid-open Patent Publication No. Hei-7-336331 discloses a digital radio communication system having means of grasping the line state on the basis of the resending frequency or the bit error rate measured and changing the coding rate of the error correction coding in accordance with the line state. Further, with respect to a code division multiple access system (CDMA system), it also discloses a system for varying the coding rate in accordance with a traffic amount and a system of measuring the line bit error rate and varying the coding rate in accordance with the measurement result. As a result, it concludes that these systems can vary the coding rate to the optimum state in accordance with the line state and achieve the optimum throughput in the line state. In this case, there is a disadvantage that it takes a long time to grasp the line state on the basis of the resending frequency, and data of a predetermined pattern must be transmitted before packet transmission in order to measure the bit error rate.